Selmayr was first appointed deputy secretary general. Then the secretary general Alexander Italianer resigned and Selmayr was within minutes appointed to the EU executive's highest administrative position.

According to Liberation, no other member of the college of commissioners than Juncker was aware of the move, including the commissioner in charge of human resources Guenther Oettinger.

The French daily also suggested that, given the timing, Selmayr did not get through the usual procedure.

Winterstein said that "each and every one" of the "stringent and difficult" steps in the procedure were passed by Selmayr and that Juncker's right-hand man was "the best candidate".

He said that the opening for the deputy secretary was published on 31 January, and that two people, including Selmayr, applied for the job . The spokesman added that Selmayr went to the commission's evaluation centre on 15 February, had an interview and evaluation on 16 February, followed by an interview with Oettinger.

He did not specify however when Selmayr was interviewed by Oettinger.

He defended Selmayr's appointment as secretary general within minutes of Italianer's resignation by the need to avoid a "gap" at the top of the institution.

"What you certainly don't want to have is the most important position being vacant for any period of time, he said.

He said that disclosing in advance that Italianer would leave "would have hampered [his] effectiveness".

Last week, however, Juncker said that he knew since appointing him in 2015 that Italianer would quit his post on 1 March 2018.

The commission spokesman argued that "decisions of such scale are taken with the required confidentiality", but declined to say since when Selmayr knew about the timing of Italianer's resignation.

He also did not deny claims that commissioners were not aware of the planned move before last Wednesday's meeting.

"There was the interview with the commissioner in charge, Guenther Oettinger. That's all I can say. And the college discussed it on Wednesday," he said.

'Cloak and dagger operation'

He added that "the college started at 9.30 and then they discussed this issue, took note of all the facts and then took their decision."

Claims that Selmayr's double appointment was a done deal are strengthened by the fact that a press conference by Juncker and Oettinger to announced the move was announced in an email sent to journalists at 9.39, while commissioners were supposed to be discussing it.

"They kept their cards close to their chest," an official noted last Wednesday, referring to the Juncker cabinet's secret preparations.

It was a "cloak and dagger operation," said Green MEP Sven Giegold, who asked for a investigation by the European Parliament.

"It is unacceptable to put important staffing decisions on the agenda of the college of commissioners without proper advance [notice]," he said in a statement, adding that "the allocation of top positions in public institutions without an open tender is a bad habit."

Central European countries say they have "nothing to hide" on rule of law issues - while justice commissioner Vera Jourova said they should agree to the Commission's controversial budget plans on rule of law conditionality.